When abrading or polishing wood, synthetic resins and some metals and alloys, notably light metals and light metal alloys, using conventional abrasive articles, such as abrasive belts, electrostatic charges are accumulated on the belt surface, and on the working piece itself. Typically, the belt may accumulate a positive charge of up to 7500 v/cm. while the wood being worked may accumulate a charge of up to -7500 v/cm. These charges have a detrimental, sometimes harmful, effect. Thus they tend to repel the particles of abraded material from some regions of the abrading machine, and from the abrasive belt itself, and these particles become suspended in the air. A large amount of the abraded particles will not be removed by the normal vacuum device downstream from the working piece. Apart from the nuisance of having the articles forming a layer of dust over the entire workshop, they also present a health and fire hazard. The dust can also adhere to the piece being worked, thereby necessitating an extensive cleaning prior to finishing.
Electrostatic charges also seem to be responsible for increased clogging of the abrasive surface. It has been shown that if the electrostatic charges can be eliminated, a grinding belt will have its lifetime, i.e., its useful time before clogging, increased at least by 50 or 100 percent, all other circumstances being equal. Electrostatic charges have been directly responsible on some occasions for accidents at a belt grinding machine where the work piece is handled manually. Less experienced workers in some cases have had serious accidents when suddenly exposed to a discharge, even if aware that such a discharge might occur.
Attempts have, of course, been made to eliminate or reduce the surface charges. As far as applicant is aware none of these attempts have been very successful.
It has been suggested to provide one or both sides of an abrasive, flexible article with a layer of conductive particles to increase heat conductivity or to reduce friction. These abrasive articles are quite effective in this respect for abrading and polishing steel and metals, but less so for treating synthetic resins, and non-effective or even harmful for the wood.
It is a common practice to use grounded metal whiskers to dissipate electrostatic charges from various moving machine parts. This has also been tried for abrasive belt grinders but with little or no success. The reason for the failure is not fully understood, but it may be assumed that the explanation lies in the structure of the abrasive surface itself. Thus the whiskers will only be able to contact the peaks of the abrasive grains, and therefore each successive row of whiskers will only be effective to discharge a small fraction of the total surface charge. Since the whiskers will of course be worn out by the abrasive surface, the use of many successive rows of whiskers is technically, as well as economically, unattractive. Another factor which prohibits the use of metal whiskers contacting the abrasive surface in belt grinders for woodwork is that abraded metal from the whiskers can contaminate the wood. The metal particles themselves would, of course, be small enough so as to be not noticeable, but many of the dyes used for coloring wood will react with metal particles to produce discoloration and spots on the treated surface.
As mentioned above, it has been proposed to provide an abrasive belt on either or both sides with a layer of metal particles in order to improve heat transfer, reduce clogging, and to reduce friction sbetween the back and the support. Such a layer or layers, applied directly on the abrasive grits or on the back, may be effective for abrading steel and some metals, but have proved able to only temporarily reduce the electrostatic surface charges on a grinding belt used for treating wood or synthetic resins and only to a small extent. Moreover, it is not permissible, as a rule, to apply metal particles to the abrasive side of grinding belts used for wood, since the particles may deposit on and contaminate the surface of the wood, as discussed above in connection with metal whiskers. Applying metal to the back of the grinding belt reduces the electrastatic charge build-up for a short time but only for a small fraction of the lifetime of the grinding belt. At least in some cases, a metalized back considerably increases the working temperature of the grinding belt, probably due to contamination with wood dust, to the extent that the backing member, which usually is made of a strong paper material, is charred and destroyed.
Despite serious attempts on behalf of many workers, the problem of eliminating an electrostatic charge built-up on the surface of conventional grinding belts used in workwodking has remained unsolved until now.